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Training for the marathon is a bit different from training for a 10 or 5 kilometer race. What are the main differences then? Well, first of all, since the marathon is such a long race, you will have to do more mileage than for a 10 km race. You will also have to get used to running at a fair pace (marathon pace) for an extended period of time, in order prepare your body for the upcoming race. Another difference is that there is really no need for short and fast intervals, since that type of running will really not be very beneficial for the race.
Running a marathon is a demanding task, and some of the problems you face are to manage dehydration, running out of fuel and fatigue. I believe that the most important workout for the marathon is the Long Run. This long run can then be made longer and longer until you can do about 2 ? - 3 hours of running, as a general upper limit. Increase the mileage gently, not more than 5 km each week. The long run can be done once every week, or combined with Marathon Pace runs so that you do the long run one week, and the Marathon Pace run the next, and so on. I will show more approaches to this in the marathon programs. The pace on the long run should be easy, about 1 - 1.5 minutes slower than your 10 km race pace, or 70 - 75% of your Max HR.
Besides this long run, the other days you run as your prefer, but try to do as much as you ready for. Decide early on your maximum mileage per week. This way you can plan the weekly amount so that you arrive at this goal mileage about 3 - 4 weeks prior to the race.
After increasing your milage up to your preferred level, you can start to incorporate speed workouts. These can be done as interval sessions and tempo runs. The pace that is most important for marathon training is Threshold Speed, but 5 km race pace intervals combine nicely with the threshold running. Threshold running is performed at about 90% of your Max HR, while the 5 km race pace intervals correlate more to 98-100% of your Max HR.
Another type of training that is of significance is Marathon Pace (MP) running. This is simply running performed at your projected marathon pace. These are typically long, harder runs of up to 2 hours, which means they should be treated as races, as far as preparation and rest goes. It is a good idea to run with somebody your own speed or even better, that can help pace you through this demanding workout. Another possibility is to enter a half-marathon and run it at MP. MP running will prepare you for the upcoming race in a very effective way, since it is actually the same speed as you want to keep in the race, except that is a bit shorter so that you can keep your mileage up.
It is of vital importance to practise drinking on your long runs. A sports drink like Gatorade or Powerade gives you a little extra boost in form of carbohydrates and electrolytes, and if you are going to use anything else than water during your race - practise this in training! I have seen many examples of people trying new approaches in a race, from Power Bars to bananas and home made potions - with the result of stomach problems and nausea. Avoid that mistake. Find out well in advance what drinks they will be serving in the race, and practise drinking this during your long runs to see how you react.Of course, the same principal applies to anything that is 'new'. Shoes, clothes, lotion, etc, do practise and get used to what you are going to use in your marathon.
How many days per week should you run? Well, this is impossible to give a general answer to. It depends on your background. In a way, I believe in the principal, "the more the better", although to a certain degree. The most important thing is to avoid injury at any cost. You need to have adequate time to recover, and doing too much can result in injury and overtraining. Individuals are different, and we can't all copy what the word-class marathoners do. If you only run once or twice a week, I would advise you to try to get up to 3 - 5 days per week of training. If you can do more and can get yourself ready for your preferred milage (remember the guideline above), you will most likely get better. I believe that long hard marathon pace workouts, combined with shorter intervals are an excellent way to prepare for a marathon.
Refer to the the marathon program for more details of how to train successfully for marathons.